The present invention relates to a transmission control device for controlling the operation of a transmission system comprising a main transmission having a torque converting device with a direct clutch and a sub-transmission having a speed change mechanism
Conventionally, a transmission system is known which combines a main transmission having a torque converting device provided with a direct clutch for effecting engagement and disengagement of the input member with output member of a torque converting portion as well as a sub-transmission capable of effecting speed change by selecting the drive mode of said speed change mechanism. For instance, such transmission systems includes a four-wheel drive automatic transmission comprising an electronic controller using as its input such as the states of running of the vehicle, including the vehicle speed and throttle opening; a hydraulic controller which inputs the output of the electronic controller; a main transmission having a plurality of speed-change steps controlled by the hydraulic controller and having a direct clutch effecting the engagement and disengagement between the input member and output member of the torque converting portion by a change-over of the hydraulic controller and the like; and a sub-transmission allowing the drive mode to be changed over between a high-speed mode and a low-speed mode In the case of such a four-wheel drive automatic transmission, even if the sub-transmission is changed over from the high-speed mode to the low-speed mode or from the low-speed mode to the high-speed mode, the engagement or disengagement of the direct clutch of the torque converting device has heretofore been controlled by fixed lock-up (direct clutch engagement) characteristics given by the vehicle speed, throttle opening, and the electronic controller regardless of the drive mode of the sub transmission.
As described above, the direct clutch of a conventional torque converting device is controlled by specific shifting characteristics, such as the vehicle speed and throttle opening, regardless of the drive mode of the sub-transmission. In consequence, if a torque converter is used as the torque converting device and the drive mode of the sub-transmission is set to the high-speed mode which is suitable for normal running and high-speed running, and if the lock-up characteristics of the direct clutch are set in such a manner as to effect an earlier lock-up so as to reduce fuel consumption at the time of normal running and high-speed running, in the performance curve of the torque converter, a lock-up is completed before the torque is increased sufficiently by a stator within a convertible range (a range in which a torque conversion is effected by the converter). As a result, in a case where the driver wishes to obtain a large torque for drive wheels by setting the drive mode of the sub transmission to a low speed, the direct clutch is locked up at an earlier point of time, thereby making it impossible to produce an increased torque for drive wheels within a convertible range. On the other hand, if the lock-up point of the lock-up characteristics of the direct clutch is set to a coupling range (a working range in which the torque ratio is 1) when the drive mode of the sub-transmission is set to the low speed, so as to produce a sufficiently increased torque for the drive wheels within a convertible range, and if the drive mode of the sub-transmission is set to a high-speed mode, and normal running and high-speed running are carried out, fuel consumption increases disadvantageously since the lock-up is delayed.